It is well-known to eliminate this type of pollutants by passing the exhaust gas through catalysts (referred to as DeNO.sub.x catalysts) intended for nitrogen oxides conversion. The known catalysts being active within a given temperature range, several catalysts having different formulations, i.e. different activity ranges, may be placed in the catalytic muffler. The sphere of action of the catalytic elements is thus extended. However, in this context, a problem comes up when the exhaust gas is not, at the level of the catalyst(s), within a temperature range for which conversion of the nitrogen oxides is sufficient.
More precisely, a problem comes up when a first catalyst is active in a first temperature range and a second catalyst is active in a second temperature range, and when these ranges are such that there is a range of temperatures between and above the two temperature ranges defined above in which the nitrogen oxides conversion will be low.
By way of illustration, the formulations used for low temperatures are of the Platinum/Alumina or Platinum/Zeolite type. The temperatures for which these catalysts are the most active are from 200.degree. C. to 250.degree. C.
Catalysts referred to as "High-temperature" catalysts are generally active between 300.degree. C. and 500.degree. C. These are for example Copper/Zeolite type catalysts.
It is of course obvious that, between these two ranges, i.e. here between 250.degree. C. and 300.degree. C., no nitrogen oxides conversion can be performed effectively. Below 200.degree. C. and above 500.degree. C., the problem is the same.
The present invention allows this type of problem to be solved.
Generally speaking, the present invention can be implemented when an exhaust line comprises such catalysts that there are temperature ranges for which NO.sub.x conversion is not sufficient.
Also known are exhaust lines such as those described for example in document DE-4,414,904, comprising a heat exchanger by means of which all the exhaust gases can be heated or cooled, as the case may be, in order to optimize conversion and to limit catalyst aging.
Document DE-4,410,022 describes an exhaust line also comprising an exchanger mounted in series, upstream from the catalyst, in order to control the catalyst temperature.
U.S. Pat. No. 3,716,344 describes a conversion catalyst provided with fins and means situated upstream in order to cool it if need be.
These known technological solutions allow to control the temperature of the gas reaching the catalyst so that the latter can effectively convert it and aging of the catalyst can be limited.
However, this type of solution does not allow fine modulation of the temperature of the gas since, in such systems, the exhaust gas flows through a heat exchanger prior to passing through the catalyst.
The present invention notably meets the problem linked with the absence of conversion for certain exhaust gas temperature ranges in which none of the catalysts is active.
An original way of solving this problem is presented hereafter.